If we wish to get the group’s icon, it would take another request to just retrieve the icon alone. Adding the iconURL to the returned JSON would be beneficial to developers working with the groups api.
We do not intend to provide information about group thumbnails in the endpoint you listed - it makes the endpoint less efficient as a whole due to the way we handle thumbnails.
The way I would implement this as a third-party developer, is to have your back-end only handle group IDs, and then wherever you show it on the front-end of your application, simply directly invoke the group thumbnail API from the front-end with the group ID given by your back-end. This way there is no need to pass the thumbnail URL from your backend.